A ball screw (ball screw apparatus) includes a nut having a spiral groove formed on the inner circumferential surface thereof, a threaded shaft having a spiral groove formed on the outer circumferential surface thereof, balls allocated in a raceway formed by the spiral groove of the nut and the spiral groove of the threaded shaft, and a ball returning passage for returning the balls from an end point of the raceway to a start point thereof, and in which the balls roll in the raceway, thereby ensuring that the nut moves relative to the threaded shaft.
In using such a ball screw, a lubricant is supplied inside the nut. When foreign matters such as dust or abrasion powder attached to the threaded shaft enter the nut, the foreign matters cause damage such as seizure of the balls or the spiral grooves. Thus, it is necessary to prevent the entry of the foreign matters into the nut. Accordingly, in order to prevent discharging of the lubricant supplied inside the nut to the outside and to prevent the entry of the foreign matters into the nut, a ring shaped seal (dust-proof component) is attached to end portions of the nut in the axis direction.
A conventional example (FIG. 4 of PTL 1) of a ball screw having a seal is illustrated in FIG. 24. In the ball screw, a ring shaped seal 40 is fixed to the inner circumferential surface of an end portion of a nut 20 in the axis direction by the use of a set screw 60 not having a head.
A method for fixing seal used in the ball screw will be described below with reference to FIGS. 25A to 25C.
First, a through hole 28 is formed in an end portion of the nut 20 in the axis direction using a drill or the like. FIG. 25A illustrates this state. Then, the through hole 28 is threaded so as to form a female screw hole 29. FIG. 25B illustrates this state. Then, the drill is made to pass through the female screw hole 29 to form a pilot hole 402 on the outer circumferential surface of a seal 40 in a state where the outer circumferential surface of the seal 40 is fitted into the inner circumferential surface of the nut 20. FIG. 25C illustrates this state.
Then, the seal 40 is fixed to the nut 20 by screwing a set screw 60 into the female screw hole 29 of the nut 20 and inserting the shaft portion of the tip of the set screw 60 into the pilot hole 402 of the seal 40. In this seal fixing method, a step of threading the through hole 28, a step of removing burrs thereafter, or the like is necessary.
A flange may be formed on the end portion in the axis direction as a seal attaching portion of the nut 20 to which the seal is attached, may not be formed thereon as with the end portion illustrated in FIG. 24. The thickness (size in the radial direction) of the seal attaching portion of the nut varies by products, because the outer diameter of the nut and the outer diameter of the flange are determined depending on customer requirements.
As illustrated in FIG. 26A, when the wall thickness TN of the seal attaching portion of the nut 20 is small, only the female screw hole 29 is formed in the seal attaching portion. As illustrated in FIG. 26B, when the wall thickness TN of the seal attaching portion of the nut 20 is large, the time required for a tapping operation is shortened by forming a drilled hole 29a on the outer circumference thereof.
The production lot of a ball screw is small and the ball screws have various sizes. Accordingly, the formation of the pilot hole 402 of the seal 40 and the through hole 28 and the drilled hole 29a of the nut 20 is carried out using a manual drilling machine without using a dedicated tool. In formation of the pilot hole 402, operations illustrated in FIGS. 27A to 27D are carried out to make the depths of the pilot holes 402 constant.
These operations will be described below with reference to a nut having a flange formed in one end portion in the axis direction and not having a flange formed in the other end portion. In this example, as illustrated in FIG. 27A, in one end portion of the nut 20 in the axis direction, a through hole 290 including an female screw hole 29 and a drilled hole 29a is formed in a flange 22.
First, as illustrated in FIG. 27B, a drill 71 not rotating is inserted into the through hole 290 until the tip of a drill bit 72 reaches the outer circumferential surface of the seal 40, and a stopper 170 for restraining forward movement of the drill 71 is attached to the drill 71.
Then, a gauge 173 having the same thickness as the depth of a pilot hole 402 to be formed in the seal 40 is disposed on the outer circumferential surface of the nut 20 around or in the vicinity of the drill bit 72, and the stopper 170 is set on the gauge 173 so as to come in contact with the gauge. Thereafter, as illustrated in FIG. 27C, the gauge 173 is extracted and the drill bit 72 is rotationally driven to form the pilot hole 402 by a drilling machining. The drill 71 drills the seal 40 from the outer circumferential surface to the inside until the forward movement thereof is restrained by the stopper 170.
Through this machining process, the pilot hole 402 is formed in the seal 40 as illustrated in FIG. 27D. By forming the respective pilot holes 402 through this machining process, the pilot holes 402 having the same depth as the thickness of the gauge can be always formed on the outer circumferential surface of the seal 40. That is, the depths of the pilot hole 402 to be formed can be managed.
This operation is carried out on the other end portion of the nut 20 in the axis direction. Since a flange is not formed in the other end portion of the nut 20 in the axis direction, only a female screw hole 29 is formed therein. The above mentioned operation is carried out by considering the female screw hole 29 as the through hole 290. The adjustment of the depth of the pilot hole 402 using the gauge 173 and the stopper 170 is carried out in correspondence with the thickness of the seal attaching portion of the nut 20.